Introduction
Communication services are often defined as: voice and telephony; video, which include video-on-demand and broadcast video and associated audio; and data communications, such as computer communications. Providing combined voice, audio/video, and data communication services is popularly referred to as providing “the triple play.”
Telephone networks with twisted pair communication lines have been traditionally designed to support voice communications. Recent developments in technology have provided the ability to support high speed data communications over traditional telephone networks. However, telephone networks using such twisted pair lines are at a significant disadvantage compared to coaxial cable (cable TV) networks. Cable connections to subscribers can support much higher bandwidths than twisted pair telephone lines. With the advent of digital subscriber line (DSL) technology, the data rates that can be support by twisted pair lines can be significantly increased. However, these rates currently fall far short of the rates that can be supported by coaxial cable systems.
To be able to compete with the cable companies in providing the triple play, the telephone companies need to further expand the capacity of the access lines and to support an audio/video broadcast capability comparable to the capability provided by the cable systems. Fiber-to-the-premise (FTTP) or fiber-to-the-home (FTTH) is the ultimate solution. However, FTTP is very expensive and will take many years to fully deploy. The telephone companies need a solution that is less expensive than FTTP and that can be deployed more quickly.
II. Telephone Networks
A telephone network includes a plurality of access networks that connect subscriber nodes to central offices (COs) and an interoffice network that interconnects the COs as illustrated in FIG. 1. A subscriber node is located on an opposite end of the access line from the CO. The illustration in FIG. 1 depicts a smaller conventional telephone network with a few COs and a small number of subscriber access lines per CO. A larger telephone network includes thousands of COs and many more access lines per CO.
In the conventional interoffice network, the COs are interconnected by fiber optic cables having multiple fibers. Each fiber supports multiple Synchronous Optical Network (SONET) channels that are time division multiplexed. In some cases, a fiber also supports multiple wavelength division multiplexed (WDM) channels that support multiple SONET channels. As shown in FIG. 1, an interoffice network typically is defined by a mesh topology. A pattern of interlocking SONET rings is superimposed on the underlying mesh topology so that data flows are constrained to follow the SONET rings. On the SONET ring, data flows in two directions, which enables a data flow to be quickly re-established if a failure occurs along the SONET ring.
Regardless of whether the interoffice network is a ring-based or a mesh-based topology, the interoffice network must have sufficient capacity to handle the data flows to and from the access network. Throughput of data through conventional interoffice network is usually limited by the capacity of the access networks. There is a desire for network capacity to be substantially increased and used more effectively in order to support the additional bandwidth requirements of integrated communication services including voice communications, data communications, audio/video-on-demand, and broadcast audio/video.
A conventional telephone network includes many subscriber nodes connected to a class 5 CO (end office) by one or more pairs of copper wires that are twisted together to reduce interference such as electromagnetic coupling to other wires. The maximum bandwidth of a signal that can be transmitted over twisted pair access lines is limited to approximately 4 KHz for typical distances between a telephone network subscriber and the corresponding telephone central office.
In some cases, the twisted pair access lines are terminated in a remote terminal in the telephone network subscribers' general geographical area, where signals on multiple subscriber lines are digitized, multiplexed onto a fiber, and sent to the CO. This latter type access is referred to as a digital loop carrier (DLC) system. COs for a common carrier wireline network support an average of approximately 12,000 subscriber access lines, which include twisted pair lines from subscriber nodes to the CO and from subscriber nodes to a DLC remote terminal. Large business enterprise customers may have fiber optic access lines running from the CO to the subscriber node as part of the SONET access ring.
III. Access Networks
In the past, bandwidth on most twisted pair access lines was limited to 4 KHz, which limits the equivalent data rate to approximately 64 Kb/s or less. Bandwidth was increased using the Integrated Services Digital Network (ISDN) basic rate interface (BRI) that supports a data rate of approximately 144 Kb/s over a twisted pair access line configured to support ISDN.
With the advent of digital subscriber line (DSL) techniques, bandwidth has been further expanded beyond the traditional 4 KHz limit and data rates in the megabit per second range can be supported over twisted pair access lines. Despite the enhanced capabilities provided by DSL, conventional access networks still limit network services as currently configured. The data rates that can be supported by DSL fall off rapidly as the length of the twisted pair lines between a CO and a subscriber node increases. Consequently, high DSL data rates are supported only for subscriber nodes that are relatively close to the CO.
The term digital subscriber line (DSL) refers to a set of techniques that enable high data rates to be transmitted over twisted pair access lines. Although there are a number of variations of DSL, two types of DSL are relevant here, asymmetric DSL (ADSL) and very high speed DSL (VDSL).
A. ADSL Access Networks
ADSL involves relatively long twisted pair access lines running from the subscriber node to the CO with a relatively wide bandwidth assigned for downstream traffic (CO to subscriber node) and a relatively narrow bandwidth assigned for upstream traffic (subscriber node to CO).
FIG. 2 shows the downstream data rates that can be supported with ADSL as a function of the length of the twisted pair access line. These rates are for the case of 24-gauge wire and upstream rates about ten percent (10%) of the downstream rates. As clearly shown by FIG. 2, the data rates supported by DSL decrease rapidly as the length of the access line increases.
A typical access line between the CO and the subscriber node, however, suffers from various degradations. For example, the use of narrower gauge wire, bridged taps, poor splices, and loading coils considerably reduces the data rates that are achieved with ADSL. Typically, 26-gauge wire is utilized at a distance up to 10,000 feet from the CO, with 24-gauge wire used for the remaining distance to the subscriber node. Bridged taps are defined as unterminated line segments off the access line, which act as delay lines and induce nulls in the frequency response. Corrosion occurs at poor splices in the lines resulting in increased attenuation. The presence of loading coils preclude the use of DSL, which are placed on long access line to improve voice quality and attenuate frequencies above 4 KHz.
Over non-loaded twisted pairs up to 18,000 feet in length, ADSL can generally support data rates sufficient for high speed Internet access for users. However, ADSL generally does not support servers that require high upstream data rates.
Video compression techniques enable video-on-demand, with image quality comparable to the quality provided by VCRs, over ADSL lines that can support a downstream data rate of approximately 1.5 Mb/s. However, higher quality video requires rates in excess of 3 Mb/s and high definition television (HDTV) requires a data rate of approximately 20 Mb/s, even with a high degree of compression. Thus, high quality video cannot be supported over many ADSL access lines, and HDTV is not compatible with ADSL. Similarly, ADSL is not compatible with the audio/video broadcast capability provided by cable TV systems. Thus, ADSL falls short of being able to support a full range of communication services.
B. VDSL Access Networks
A second variation of DSL is a very-high-speed DSL (VDSL), which is similar to ADSL, except that VDSL involves relatively short access lines that run from the subscriber node to a remote terminal in the subscribers' general geographic area. VDSL supports higher data rates than ADSL. Typically, VDSL is asymmetric with downstream bandwidth and data rates higher than the upstream bandwidth and corresponding data rates. VDSL is capable of supporting higher data rates than other variations of DSL. VDSL twisted pair access lines from subscriber nodes are terminated in a remote terminal in the subscribers' general geographic area. This remote terminal is then connected to a CO by a fiber optic line. This approach greatly shortens the length of the twisted pair lines and enables VDSL to support high data rates as illustrated in FIG. 3.
For an access line length of 3,000 feet or less, VDSL can provide data rates greater than 20 Mb/s, which is sufficient to support an HDTV channel or several conventional TV channels. However, VDSL by itself can support only a very limited audio/video broadcast capability and cannot support a broadcast capability comparable to that provided by cable TV systems.